It is well known in the art that a sound produced within a reflective environment may traverse many diverse paths in reaching a receiving transducer. In addition to the direct path sound, delayed reflections from surrounding surfaces, as well as extraneous sounds, reach the transducer. The combination of direct, reflected and extraneous signals result in the degradation of the audio system quality. These effects are particularly noticeable in environments such as classrooms, conference rooms or auditoriums. To maintain good quality, it is a common practice to use microphones in close proximity to the sound source or to use directional microphones. These practices enhance the direct path acoustic signal with respect to noise and reverberation signals.
There are many situations, however, in which the location of the source with respect to the electroacoustic transducer cannot be controlled. In conferences involving many people, for example, it is difficult to provide each individual with a separate microphone or to devise a control system for individual microphones. One technique disclosed in U.S. Pat. No. 4,066,842 issued to J. B. Allen, Jan. 3, 1978 utilizes an arrangement for reducing the effects room reverberation and noise pickup in which signals from a pair of omnidirectional microphones are manipulated to develop a single, less reverberant signal. This is accomplished by partitioning each microphone signal into preselected frequency components, cophasing corresponding frequency components, adding the cophased frequency component signals, and attenuating those cophased frequency component signals that are poorly correlated between the microphones.
Another technique disclosed in U.S. Pat. No. 4,131,760 issued to C. Coker et al Dec. 26, 1978 is operative to determine the phase difference between the direct path signals of two microphones and to phase align the two microphone signals to form a dereverberated signal. The foregoing solutions to the noise and dereverberation problems work as long as the individual sound sources are well separated, but they do not provide spatial volume selectivity. Where it is necessary to conference a large number of individuals, e.g., the audience in an auditorium, the foregoing methods do not adequately reduce noise and reverberation since these techniques do not exclude sounds from all but the location of a desired source. It is an object of the invention to provide improved audio signal processing which reduces interference in a noisy, reverberant environment.